Buggy code:
public abstract class AbstractDOMParser extends AbstractXMLDocumentParser { protected static final String CREATE_ENTITY_REF_NODES = "http:////apache.org//xml//features//dom//create-entity-ref-nodes" ; protected static final String NAMESPACES = ( Constants . SAX_FEATURE_PREFIX ) + ( Constants . NAMESPACES_FEATURE ) ; protected static final String INCLUDE_COMMENTS_FEATURE = Constants . INCLUDE_COMMENTS_FEATURE ; protected static final String CREATE_CDATA_NODES_FEATURE = Constants . CREATE_CDATA_NODES_FEATURE ; protected static final String INCLUDE_IGNORABLE_WHITESPACE = "http:////apache.org//xml//features//dom//include-ignorable-whitespace" ; protected static final String DEFER_NODE_EXPANSION = "http:////apache.org//xml//features//dom//defer-node-expansion" ; protected static final String NORMALIZE_DATA = ( Constants . XERCES_FEATURE_PREFIX ) + ( Constants . SCHEMA_NORMALIZED_VALUE ) ; protected static final String DOCUMENT_CLASS_NAME = "http:////apache.org//xml//properties//dom//document-class-name" ; protected static final String DEFAULT_DOCUMENT_CLASS_NAME = "org.apache.xerces.dom.DocumentImpl" ; protected static final String CURRENT_ELEMENT_NODE = ( Constants . XERCES_FEATURE_PREFIX ) + ( Constants . CURRENT_ELEMENT_NODE_PROPERTY ) ; private static final boolean DEBUG_ENTITY_REF = false ; private static final boolean DEBUG_EVENTS = false ; protected boolean fCreateEntityRefNodes ; protected boolean fIncludeIgnorableWhitespace ; protected boolean fIncludeComments ; protected boolean fCreateCDATANodes ; protected boolean fNormalizeData = true ; protected Document fDocument ; protected DocumentImpl fDocumentImpl ; protected String fDocumentClassName ; protected DocumentType fDocumentType ; protected Node fCurrentNode ; protected CDATASection fCurrentCDATASection ; protected StringBuffer fInternalSubset ; protected boolean fDeferNodeExpansion ; protected boolean fNamespaceAware ; protected DeferredDocumentImpl fDeferredDocumentImpl ; protected int fDocumentIndex ; protected int fDocumentTypeIndex ; protected int fCurrentNodeIndex ; protected int fCurrentCDATASectionIndex ; protected boolean fInDTDExternalSubset ; protected boolean fInDocument ; protected boolean fInCDATASection ; private QName fAttrQName = new QName ( ) ; protected AbstractDOMParser ( XMLParserConfiguration config ) { } protected String getDocumentClassName ( ) { <START_BUG> return documentClassName ; <END_BUG> } protected void setDocumentClassName ( String documentClassName ) { } public Document getDocument ( ) { } public void reset ( ) throws XNIException { } public void startGeneralEntity ( String name , XMLResourceIdentifier identifier , String encoding , Augmentations augs ) throws XNIException { } public void textDecl ( String version , String encoding , Augmentations augs ) throws XNIException { } public void comment ( XMLString text , Augmentations augs ) throws XNIException { } public void processingInstruction ( String target , XMLString data , Augmentations augs ) throws XNIException { } public void startDocument ( XMLLocator locator , String encoding , Augmentations augs ) throws XNIException { } public void xmlDecl ( String version , String encoding , String standalone , Augmentations augs ) throws XNIException { } public void doctypeDecl ( String rootElement , String publicId , String systemId , Augmentations augs ) throws XNIException { } public void startElement ( QName element , XMLAttributes attributes , Augmentations augs ) throws XNIException { } public void characters ( XMLString text , Augmentations augs ) throws XNIException { } public void ignorableWhitespace ( XMLString text , Augmentations augs ) throws XNIException { } public void endElement ( QName element , Augmentations augs ) throws XNIException { } public void endPrefixMapping ( String prefix , Augmentations augs ) throws XNIException { } public void startCDATA ( Augmentations augs ) throws XNIException { } public void endCDATA ( Augmentations augs ) throws XNIException { } public void endDocument ( Augmentations augs ) throws XNIException { } public void endGeneralEntity ( String name , Augmentations augs ) throws XNIException { } public void startDTD ( XMLLocator locator , Augmentations augs ) throws XNIException { } public void endDTD ( Augmentations augs ) throws XNIException { } public void startExternalSubset ( Augmentations augs ) throws XNIException { } public void endExternalSubset ( Augmentations augs ) throws XNIException { } public void internalEntityDecl ( String name , XMLString text , XMLString nonNormalizedText , Augmentations augs ) throws XNIException { } public void externalEntityDecl ( String name , String publicId , String systemId , String baseSystemId , Augmentations augs ) throws XNIException { } public void unparsedEntityDecl ( String name , String publicId , String systemId , String baseSystemId , String notation , Augmentations augs ) throws XNIException { } public void notationDecl ( String name , String publicId , String systemId , String baseSystemId , Augmentations augs ) throws XNIException { } public void elementDecl ( String name , String contentModel , Augmentations augs ) throws XNIException { } public void attributeDecl ( String elementName , String attributeName , String type , String [ ] enumeration , String defaultType , XMLString defaultValue , XMLString nonNormalizedDefaultValue , Augmentations augs ) throws XNIException { } }

Fix:
return fDocumentClassName ;